1. Field of the Invention
The invention relates in general to a filter, and more particularly to a bandwidth-adjustable filter.
2. Description of the Related Art
In wireless communication systems, according to a common modulation mechanism, when in-phase signals (I-channel) and quadrature-phase signals (Q-channel) have different 3 dB bandwidth (ω0) in the filter, these two kinds of signals cannot match in phase. When the signals in the I-channel and in the Q-channel have a phase difference not equal to 90 degrees, the signal constellation is distorted and the bit error rate is thus increased. Therefore, in order to maintain system stability, it is very important that the filter has accurate 3dB bandwidth.
Referring to FIG. 1, a schematic diagram of a filter disposed in a wireless communication system is shown. The filter 100 has a closed-loop gain H0 equal to −R3/R2, and has a 3 dB frequency ω0 represented by
                              ω          0                =                  1                                    R              ⁢                                                          ⁢              1              ×              R              ⁢                                                          ⁢              3              ×              C              ⁢                                                          ⁢              2              ×              C              ⁢                                                          ⁢              1                                                          Eq        .                                  ⁢        1            
When the gain H0 is −1, Eq.1 can be represented by
                                          ω            0                    =                      1                                                            m                  ×                  n                                            ×              R              ×              C                                      ⁢                                  ⁢                                                            wherein                            ⁢                                                          ⁢              R              ⁢                                                          ⁢              1                        =            mR                    ,                                          ⁢                                    R              ⁢                                                          ⁢              2                        =            R                    ,                                          ⁢                                    C              ⁢                                                          ⁢              1                        =            C                    ,                                          ⁢                                    C              ⁢                                                          ⁢              2                        =                          nC              .                                                          Eq        .                                  ⁢        2            
As shown in Eq. 2, by selecting suitable values R and C, the 3 dB frequency ω0 can reach the expected value. In the modulation mechanism of FIG. 1, a capacitive array method is used to achieve the modulation of 3 dB frequency by adjusting the valid capacitance C. However, the conventional modulation mechanism has a minimum adjustable scale only about 2˜3%, which cannot meet the requirement of high accuracy for the wireless communication system.